Method for decreasing the softening temperature and improving the electrical conductivity of high conductivity oxygen-free copper



United States Patent Oflice 3,525,605 Patented Aug. 25, 1970 US. Cl. 75-76 2 Claims ABSTRACT OF THE DISCLOSURE A method for decreasing the softening temperature and improving the electrical conductivity of oxygen-free copper by adding lanthanides (elements No. 57-7l) or their alloys to the copper in connection with the smelting thereof in amounts ranging from grams to 1000' grams per ton copper or 0.001-0.1 percent depending on the quantity of impurities in the copper and on its intended use. An example of a lanthanide element which is effective is cerium or its alloy, the preferred cerium content is 0.002-0.05 percent of copper.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a method for decreasing the softening temperature and improving the electrical conductivity of oxygen-free high conductivity copper.

Description of the prior art Oxygen-free copper of high conductivity differs from the oxygen containing (tough pitch) copper generally used as electrical conductors in that it possesses a greater toughness, a better weldability and a better hydrogen annealing resistance. Thus its technical utility value generally is greater than that of the tough pitch copper. The softening temperature of this quality of copper however is higher than that of good tough pitch copper, in other Words, in order to get the material that has been worked into a hard condition to become soft again, a higher annealing temperature has to be used than with the tough pitch copper. Low softening temperature is required in some special cases such as when manufacturing magnet wires of small diameter, the temperature resistance of the coating enamel then restricting the permissible annealing temperature. Low annealing temperature is also required when annealing copper wire spools because of stickiness i.e. the contact welding of adjacent coils is to be avoided. This restricts the applicability of oxygen-free copper in these cases, although on the other hand just in fine wire draw ing its greater toughness would be to great advantage in reducing wire breaks and production interruptions.

Dissolved impurities in metal increase the softening temperature and decrease the electrical conductivity. Refining high conductivity copper is generally carried out electrolytically, so that the quantity of impurities can be kept fairly low. There is however one impurity, viz sulphur, the removal of which to the desired extent is particularly difficult in this connection, since the electrolyte being used in the refining process contains plenty of sulphur compounds (sulphuric acid, copper sulphate). The amounts of sulphur that remain in solid solution in the conventional copper treatment steps are very small, e.g. at 700 C. only about 10 grams per ton (0.001%) are dissolved, but still they have an essential influence particularly on the softening temperature. Were it desired to decrease the softening temperature of oxygen free copper to the same level as that of tough pitch copper by means of reducing its sulphur content, it would be necessary to reduce it to the range of from 1 to 2 grams per ton. This must however be considered as an unreasonable requirement hearing in mind that the permissible sulphur content of technical copper of the highest quality viz copper for electronic devices according to international standards is 20 grams per ton. Thus it has become necessary to look for other ways to solve the problem.

One way is to try to bring the sulphur, without reducing its content in the copper, into a form where its detrimental effects are eliminated. Another way specifically for decreasing the softening temperature, is to facilitate the most difficult stage of the softening process, i.e. the beginning of the recrystallization. In principle this can be done e.g. by means of increasing the number of nucleation sites in the micro structure of the metal.

SUMMARY OF THE INVENTION The above mentioned improvements have now been achieved by means of the method according to this invention.

The principle of the method according to the invention is that the dissolved sulphur contained in the copper is brought into an inefficient form by means of binding it with a suitable added ingredient and at the same time by utilizing the resulting precipitates as facilitating the recrystallization. It is of course required that the added ingredient must be such that its use does not have any detrimental elfect on other properties of the copper.

As an added ingredient lanthanides (elements No. 57- 71) especially cerium or cerium-containing alloys can be utilized. It has been experimentally proved that the addition of a lanthanide element such as cerium or its alloys decreases the softening temperature of oxygen-free cop per by an amount ranging from 50 to C. and at the same time stabilizes the properties of the copper so that both the softening temperature and the electrical conductivity become in practical conditions independent of the heat treatment preceding the cold working step and of the hot working temperature. A preferred cerium content is in the range from 2 to 500 grams per ton (from 0.002 to 0.05%) depending on the purity of the basis material and on the intended application.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following a series of experiments is set forth for better understanding of the invention.

The alloys melted by means of a vacuum induction furnace. The charges were about 25 kilograms. The cerium was added as a master alloy (5% of Jcerium). The molten material was cast in a water cooled chill mould. The ingots were machined into 97 mm. diameter billets, which subsequently were hot extruded into 18 mm. diameter rods. Next followed drawing to 5.7 mm. diameter and finally (softening) annealing at 500 C. for 2 hours.

This 5.7 mm. diameter wire was cut into suitable lengths (10 pieces of each mixture) which were annealed in a salt bath at 900 C. for 1.5 hours and water quenched. After this annealing the following heat treatments (Table I) were carried out followed by water quenching.

TABLE I Reference Temp. C.) Time (h 3 The analyses of the tested alloys were as follows (grams per ton).

TABLE 11 "IIIIIIIIIIIIIIIIIIIII "i "ii "156 "2i2 11 10 19 19 1 1 1 1 1 NOTE.The Ce-contents according to the charging.

A hydrogen annealing embrittlement test was carried out to check that the copper was oxygen free.

The results were as follows:

TABLE III Alloy: Bending values 1 15,12 2 15,13 3 11,1. 4 16,14 5 15,14 6 15,15

The bending values meet requirements set upon oxygen-free copper.

After the heat treatment the wires were cold drawn to 2 mm. diameter, the degree of deformation being 87.7%. The electrical conductivity of these wires was measured by double observations and their /z-softening tempera- The determination of the /2-softening temperature was carried out in the conventional manner. The results are presented in Table V.

TABLE V.-/-SOFTENING TEMPERATURE C.)

A B C D E The result of the investigation was according to the presupposition. Nothing was found to indicate that any of the physical or mechanical properties of the copper should have deteriorated. Instead the recrystallization temperature can be significantly decreased and the electrical conductivity improved by virtue of the Ce-addition and in addition its toughness remains particularly good at high temperatures.

What is claimed is:

1. A method of decreasing the softening temperature and improving the electrical conductivity of high conductivity oxygen free copper having sulphur in ordinary amounts, comprising the steps of reducing oxygen copper to the molten state, adding a lanthanide element to the molten copper in an amount equal by weight to about four times the amount of sulphur originally present, and solidifying the resultant molten mass.

2. The method according to claim 1, said lanthanide element being cerium.

References Cited UNITED STATES PATENTS RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75l53 

